The popular 5:2 diet took off in the UK after Michael Mosley presented the concept in a documentary on the BBC2 titled “Eat, Fast and Live Longer.”

So, what exactly is the 5:2 fasting diet, and does it work? It is it safe and, most importantly, is it hard? Let’s review.

What is the 5:2 diet?

5:2 diet is an alternate fasting diet that refers to eating ordinarily five days a week, and for two days (either two continuous or two unrelated days) you restrict your calories to 500kcal for women, 600 kcal for men.

Does it work?

When this diet first came out in 2012, there was little scientific validation to let us look at it critically and determine if it really works, or if it is safe. The UK National Health Service labeled it a “fad diet” at the time. But, is it? Consider the science since that time:

In the medical and scientific journal “Obesity Science & Practice (2016), a study called a “Meta Analysis” was published. A Meta Analysis is not a review, per say, but a compilation of similar clinical studies on the same topic to weed out studies that are misleading, as well as using the power of statistics to determine a scientific answer to a question.

They were able to compile a group of studies that included 1,193 participants (which is a big grouping for a weight loss comparison) comparing alternate day fasting (similar to 5:2 diet)to very low calorie diets (standard diet).

The results were quite interesting. Yes, the participants in the very low calorie diets lost slightly more than the participants in the alternate day fasting diets. BUT! They lost less fat and much more bone and muscle. The alternate day fasting diet participants lost more fat mass, and preserved the rest of their body. This is of course, ideal for a diet.

Is it safe?

The alternate day fasting diet (normal eating five days per week and calorie restricted two days per week) is more effective at losing fat and preserving bone and muscle than the very low calorie diets (such as cutting down to 1,200 – 1,800 kcal per day), so of course this is safe.

In the study “Diabetes Research and Clinical Practice (2016),” they evaluated the 5:2 diet in Type 2 Diabetes. This is a common condition in the United States, with over 9% of adults suffering from this condition. Type 2 Diabetes is generally the result of obesity, which effects over 35% of the adult population. The trend from overweight, obese Type 2 Diabetes is growing with over 50% of adults falling into one or all of those categories.

In this study, they found that not only was the 5:2 diet safe in diabetics, but it resulted in better blood sugar control, and similar weight loss to very low calorie diets at the end of the three month study.

Just to be clear, this was comparing usual diabetic very low calorie diets of about 1,200 kcal per day to alternate day fasting of about 500 kcal per day for two days a week; and habitual eating on the other days. So, exercising willpower for two days a week was as effective as exercising willpower for seven days a week.

In the August 2016 “Frontiers in Physiology,” a study was published that compared alternate day fasting (similar to the 5:2 diet) to a healthy heart diet daily in overweight or obese adults.

At the end of the three month study they found similar improvements in weight as well as blood lipids (such as cholesterol and LDL). But, they also found that there was less weight gain for the following one year on the alternate day fasting diet than a typical diet, and there was a better outcome in the circulatory system.

In one of the studies with obesity and diabetic participants, they kept people in a hospital for one week to see if they got sick on alternate day fasting and measured blood work for alterations. There were no issues.

In fact, all of the studies that followed people beyond the few months of the study period discovered less weight gain in alternate day fasting vs zero calorie diets.

This brings us to the final question:

Is it hard?

In the clinical studies, the participants found that alternate day fasting was generally easier than daily adherence to very low calories. This can be demonstrated in the fact that they were able to keep the weight off more effectively, and they lost more fat. Another study, which was much more aggressive than the 5:2 diet (eating normal for five days and cutting down to 500-600 kcal for two days) was published in the 2016 “Obesity Science & Practice” journal where they put people on every other day low calorie diets, and compared that to cutting down calories by 400 kcal a day. In this study, they again found better outcomes in fat free mass preservation, more fat loss, and better long-term results.

Yes. The 5:2 diet works. It is safe. It is hard like all diets, but easier than just cutting down calories.

Everybody in my family growing up suffered from obesity. So, I have always been very aggressive on weight control and my eating and fitness. I have tried virtually every diet so I can see how it feels in order to describe it to my patients. My habitual eating is pretty much the “Mediterranean Diet,” which is mostly vegetables, some fruits and nuts, and occasional fish.

When I have done fasting, I was surprised as to how easy it was. I recently returned from a trip to Europe and Africa, where I ate restaurant food the entire time. After coming home, I did a two day fast to clean out my body. And not two days in a row, but two out of three days. It was pretty easy. I was able to exercise with my usual vigor, and the hunger only was difficult at the beginning and end of the day, and that was when I looked at where I keep snacks.

The 5:2 diet is an entirely safe, effective, and reasonably easy way to lose weight. I would like to encourage people to continually work towards eating very clean and healthy, and believe the 5:2 diet is a good way to start. It has better long-term success than the traditional heart healthy, diabetic and very low calorie diets. And something happens when we hit goals and successes. The next goal and success is that much closer.